This invention relates to a step feed drilling machine in which one drilling process is carried out, instead of being performed in a single stroke, by gradually deepening a bore-depth by means of repeating reciprocative movement of a tool for a plurality of times before completing the drilling process.
More particularly this invention is concerned with a drilling machine provided with pneumatic feed means for reciprocating a quill, feed control means for engaging with the advancing quill to control feed speed of the quill, and control means for controlling the reciprocative operation of the quill, wherein the tool is moved from a start position firstly at a rapid feed speed as far as a position slightly short of a work to be processed, moved forwards for a drilling process at a process feed speed, retracted once in a backward direction at a rapid speed due to the action of the control means, and moved again at the rapid feed speed as far as a nearby end position of the previous drilling process in order to resume another drilling process.
As a drilling machine of this type there has conventionally been one provided with a dog bar attached to the quill parallelly to the moving direction of the quill for being capable of integrally moving therewith, a speed change switch for switching the moving speed of the quill, and a dog temporally fixed on the dog bar, being biased by suitable biasing means, in an engageable manner with the speed change switch, wherein, after the engagement of the dog which has advanced with the quill with the speed change switch until the quill is retracted by the action of step feed control means, the dog is relatively moved to the dog bar resisting the action of the biasing means so that a distance the quill will be moved at the rapid feed speed again after having once been retracted may be longer than the distance of the previous advancing.
In this type structure, however, both a solenoid operated directional control valve for changing forward and backward motion of the quill and a solenoid operated directional control valve for changing the speed in the forward motion of the quill are required, inevitably enlarging the apparatus as a general tendency through the increase of the solenoid operated directional control valves. Furthermore, for returning the dog to the original position after completing one drilling process a specific returning device is needed, which in turn requires another control means for the returning device. The control means itself will be very complicated in addition to the above-mentioned solenoid operated directional control valves.
Another conventional type is provided with a pneumatic cylinder for reciprocating the quill, a liquid cylinder which comprises a cylindrical housing parallelly disposed to the quill and a piston rod with a projecting portion projecting from one end of the housing, for controlling the moving speed of the quill by engaging with the quill while the same is in forward motion, and control means for controlling the pneumatic cylinder so as to make the quill repeatedly reciprocate, whereby the piston rod which has been advanced with the quill is, everytime when the quill is retracted after a forward motion by the pneumatic cylinder, retained at the advanced position, and the quill is, when it is advanced again after the retraction, fed at a rapid feed speed as far as the position the piston rod is retained, so that the distance of the rapid feed of the quill may be gradually increased.
Most drilling machines belonging to this type are designed such that a liquid is positively supplied to the retracting side of the liquid cylinder for restoring the piston rod of the feed control means to the original position after the completion of one cycle of process. In this case, too, a solenoid operated directional control valve for changing the moving direction of the quill and another solenoid operated directional control valve for restoring the piston rod are required to be installed. This type is also not free from disadvantages similar to the previous example.